Recently, forged crankshafts for which thermal refining treatment is omitted have been provided. Thermal refining treatment is a quenching and tempering treatment for improving mechanical properties of steel such as strength. Hereafter, a forged crankshaft for which thermal refining treatment is omitted is also referred to as a “non-heat treated forged crankshaft.”
The steel material constituting a non-heat treated forged crankshaft generally contains vanadium (V). A non-heat treated forged crankshaft is produced by hot forging the non-heat treated steel and allowing it to cool in the atmosphere. The microstructure of the steel material constituting the non-heat treated forged crankshaft is a ferrite-pearlite structure. Vanadium (V) remains in steel in the form of fine carbides and increases the strength and hardness of steel. In short, even without thermal refining treatment, a non-heat treated forged crankshaft containing vanadium has excellent strength and hardness. However, since vanadium is expensive, the manufacturing cost of non-heat treated forged crankshaft becomes high. Therefore, there is a need for a non-heat treated forged crankshaft having excellent strength and hardness even without containing vanadium.
Further, wear resistance is required for the surface of the forged crankshaft. A crank pin of the forged crankshaft is inserted into a large-end portion of a connecting rod. As the crankshaft rotates, the crank pin rotates via the inner surface of the large-end portion of the connecting rod and a slide bearing. Therefore, the surface of the crank pin is required to have excellent wear resistance.
JP2000-328193A and JP2002-256384A disclose a non-heat treated steel which is intended for improving wear resistance without addition of vanadium.
The non-heat treated steel for hot forging disclosed in JP2000-328193A has a ferrite-pearlite microstructure. Further, in the non-heat treated steel for hot forging, Si and Mn dissolve in ferrite thereby strengthening the ferrite. This is intended for improving wear resistance.
The non-heat treated steel for crankshaft disclosed in JP2002-256384 has a microstructure dominantly made up of pearlite with a pro-eutectoid ferrite fraction of less than 3%, and contains sulfides inclusions having a thickness of not more than 20 μm. Further, Si content is not more than 0.60%, and Al content is less than 0.005%. This is intended for improving wear resistance and machinability.
Meanwhile, to improve the wear resistance of a forged crankshaft, in general, a forged crankshaft is subjected to a surface hardening heat treatment. The surface hardening heat treatment includes, for example, an induction hardening treatment and a nitriding treatment. By an induction hardening treatment, a quenched layer is formed on the surface of a forged crankshaft. Moreover, by a nitriding treatment, a nitrided layer is formed on the surface of a forged crankshaft. The quenched layer and the nitrided layer have high hardness. Therefore, the wear resistance of the surface of the forged crankshaft is improved.
However, performing surface hardening heat treatment will result in an increase of manufacturing cost. Therefore, there is a need for a non-heat treated forged crankshaft which has excellent wear resistance even if vanadium is not contained and the surface hardening heat treatment is omitted.
A forged crankshaft manufactured by using a non-heat treated steel disclosed in JP2000-328193 and JP2002-256384 may exhibit a decline in wear resistance when the surface hardening heat treatment is omitted.